2.9 dirty shading on model in object mode when subdividing?
There are several problems. Here a short summary ;-)

apply the scale in Object Mode (Ctrl+A) as recommended by JacksonPro in the comments.

disable Normals → Auto Smooth in object data properties

remove/disable the modifiers

delete half of the mesh and add a Mirror Modifier

Fix the oral cavity. There went something wrong. The bottom of the cavity is connected to the upper lip and vice versa. I've added a seam, selected the cavity with L up to the seam, and separated the mesh to get an overview. It's twisted.


Fix the toplogy/edge flow. (Especially the mouth needs a horizontal edge loop to separate the lips. I've added an edge loop (Ctrl+R) to the corner of the mouth, and used the knife (k) to connect it to the next pole. Then I've done a lot of dissolve edges (X) and knife cuts to create a clean topology. It might be not perfect but to give you an idea. You can add a few more edge loops to the eyes (Ctrl+R) around the eyes if you want to animate the eye lids.


Smooth out the mesh in Sculpt move with the Smooth brush at low strength (~0.18)

Apply the mirror modifier. Then you can start work on the eye. This should be fairly easily done with the Deformation or Grab brush in Sculpt mode. Have fun!